Digital bit streams are transmitted and received in many digital communications systems including but not limited to data storage systems and wireless and wired data communications systems. For wireless communications, there is currently an evolution from analog communications to digital communications. Speech is represented by a series of bits that are modulated and transmitted from a base station to a radiotelephone. The radiotelephone demodulates the received waveform to recover the bits, which are then converted back into speech. There is also a growing demand for data services, such as e-mail and Internet access, which generally use digital communications.
There are many types of digital communications systems. Traditionally, Frequency-Division-Multiple-Access (FDMA) is used to divide the spectrum into a plurality of radio channels corresponding to different carrier frequencies. These carrier frequencies may be further divided into time slots, referred to as Time-Division-Multiple-Access (TDMA), as is the case in the D-AMPS, PDC, and GSM digital cellular radiotelephone systems. Alternatively, multiple users can use the same radio channel using spread spectrum techniques such as Code-Division-Multiple-Access (CDMA).
Direct-Sequence (DS) spread-spectrum modulation is commonly used in CDMA systems, in which each information symbol is represented by a number of “chips.” Representing one symbol by many chips gives rise to “spreading”, as the latter typically uses more bandwidth to transmit. The sequence of chips is referred to as a spreading code or signature sequence. At the receiver, the received signal is despread using a despreading code, which is typically the conjugate of the spreading code. IS-95 and J-STD-008 are examples of DS CDMA standards.
With DS CDMA systems, coherent Rake reception is commonly used. The received signal is despread by correlating to the chip sequence, and the despread value is weighted by the conjugate of a channel coefficient estimate, removing the phase rotation of the channel and weighting the amplitude to indicate a soft or confidence value. When multipath propagation is present, the amplitude can vary dramatically. Multipath propagation can also lead to time dispersion, which can cause multiple, resolvable echoes of the signal to be received. Correlators are aligned with the different echoes. Once the despread values have been weighted, they are summed. This weighting and summing operation is commonly referred to as Rake combining. Rake combining is described, for example, in U.S. Pat. No. 5,237,586 to the present inventor entitled “Rake Receiver with Selective Ray Combining” and U.S. Pat. No. 5,305,349 to Dent entitled “Quantized Coherent Rake Receiver”, the disclosures of both of which are hereby incorporated herein by reference in their entirety.
A conventional digital communications system 100 is shown in FIG. 1. Digital symbols are provided to a transmitter 101, which maps the symbols into a representation appropriate for the transmission medium or channel such as a radio channel 103, and couples the signal to the transmission medium via a transmit antenna 102. The transmitted signal passes through the medium 103 and is received at a receive antenna 104. The received signal is passed to receiver 105. The receiver 105 includes a pre processor such as a radio processor 106, a baseband signal processor 110, and a post processor 112.
The radio processor 106 tunes to the desired band and desired carrier frequency, then amplifies, mixes, and filters the signal down to baseband. The signal is sampled and quantized, ultimately providing a sequence of baseband received samples. Since the original radio signal has in-phase (I) and quadrature (Q) components, the baseband samples may also have I and Q components, giving rise to complex baseband samples.
The baseband processor 110 is used to detect the digital symbols that were transmitted. It may produce soft information as well, which gives information regarding the likelihood of the detected symbol values.
The post processor 112 performs functions that may depend highly on the particular communications application. For example, it may use the soft detected values to perform forward error correction decoding or error detection decoding. It also may convert digital symbols into speech using a speech decoder.
Coherent detection generally uses an estimation of how the symbols were modified by the transmitter 101, the channel 103, and/or the radio processor 106. As discussed previously, the transmission medium may introduce phase and amplitude changes in the signal, for example as a result of multipath propagation. The signal may also have become dispersed, giving rise to signal echoes. Each echo may have an associated phase and amplitude, represented by a complex channel coefficient. Each echo also may have an associated delay. Coherent demodulation generally uses an estimation of these delays and coefficients. Typically, the channel is modeled as discrete rays, with channel coefficients assigned to the different delays.
In the IS-95 system, the forward link or down link (base station to radiotelephone) includes a shared pilot channel. The transmitted signal on this pilot channel generally is known, and the power is typically much higher than the traffic or information bearing channel. Thus, the pilot channel generally provides a reference signal for use in channel estimation.
When using the pilot channel for channel estimation, the resulting channel estimate is proportional to the square root of the power on the pilot channel. Ideally, the channel estimate should be proportional to the square root of the power on the traffic channel. When performing channel estimation on different signal echoes from the same base station, this proportional difference generally is not a problem, since all channel coefficients may differ by the same proportional amount.